An active bonnet is taken to mean a bonnet which is automatically raised in the event of an accident by one or more actuators to increase the spacing between the bonnet and parts of the motor vehicle which are located therebelow and are difficult to deform, and to thus provide a pedestrian possibly impacting against the bonnet with an extended braking path. This is intended to avoid the pedestrian colliding against the non-deformable body parts at a speed which may lead to life-threatening injuries.
Various pyrotechnic actuators have already been proposed for this purpose which use gas generators, such as are also known from airbags, to drive the bonnet movement. These pyrotechnic actuators have an advantageously short response time as they are generally electrically ignited and therefore solid parts do not have to be accelerated in a time-consuming manner to activate them, and they achieve a powerful acceleration of the bonnet so it reaches its raised position in the shortest time. However, it is disadvantageous that the conventional pyrotechnic actuators are not reversible. In other words, they are only suitable for a single use and then have to be exchanged. A faulty triggering of the actuators therefore makes an expensive stay in a workshop necessary every time. As it is hardly possible to avoid triggering of the actuators in the event of an accident, if the bonnet is unaffected by the events of the accident, the cost of repair of a vehicle equipped with pyrotechnic actuators is generally higher after an accident than those of a vehicle without these actuators.
A reversible actuator for a motor vehicle is known from DE 103 08140 B3, which is driven mechanically by spring force. An actuator of this type makes it apparently simple, after it has been triggered, to bring the bonnet back into its normal position from which a renewed triggering can take place, but the spring has to have large dimensions because of the very short lifting times so reversal of the spring is possible only with the aid of a gearing down mechanism. A further problem emerges from the fact that the tensioned springs press with a high force against an obstacle which, on activation of the actuator has to be moved extremely rapidly. A high pressure of the spring against the obstacle makes a correspondingly high driving force to remove the obstacle necessary, the build up of which severely delays the actuating movement of the actuator.
A further problem of conventional active bonnets is that the known actuators are in a position to sharply accelerate the bonnet but, as soon as the actuators reach their stop, the bonnet is decelerated still more abruptly than it was previously accelerated, and made to strongly vibrate. If a pedestrian impacts against a region of the bonnet between two actuators while said region is deflected downward, the spacing between this region of the bonnet and hard parts of the vehicle located below is sharply reduced, and there is a risk that the distance available for the deceleration of the pedestrian is not sufficient to avoid a collision against these hard vehicle parts.
In view of the foregoing, at least one object of the present invention is to provide an actuator for an active bonnet which is suitable for driving a plurality of lifting movements of the bonnet without making a stay in a workshop necessary after each of these lifting movements, and which thus avoids the disadvantages of a mechanical drive. In addition, other objects, desirable features, and characteristics will become apparent from the subsequent summary, detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.